Abstract:

An atomic clock comprises helium 3 plasma as measurement medium, which is
taken to the plasma state to exploit the metastable state of the material
and the levels of the hyperfine structure, the lifetime of which is long
and which thus enable an easier measurement than the excitations of
gaseous atoms.

Claims:

1. Atomic clock comprising a cell filled with a measurement medium, a
first device for exciting particles of the measurement medium up to a
higher energy level, a system collecting a light energy frequency
returned by the measurement medium on leaving the higher energy level,
said light energy frequency being exploited to give a time measurement, a
device for applying magnetic fields comprising at least one essentially
static magnetic field and means for controlling said device to adjust the
magnetic fields, characterised in that the measurement medium comprises
helium 3 plasma, and it is provided with a second exciter device to give
rise to helium 3 plasma from gaseous helium 3.

2. Atomic clock according to claim 1, characterised in that the second
exciter device is a power radiofrequency wave generator.

3. Atomic clock according to claim 2, characterised in that the
radiofrequency waves are between 20 MHz and 30 MHz.

4. Atomic clock according to claim 2, characterised in that the
radiofrequency waves have a power of 1 W for a quantity of helium 3 of
100 mm3 at a pressure of around 0.1 torr.

5. Atomic clock according to claim 1, characterised in that the higher
energy level, from which the measurement medium returns the light energy
frequency exploited to give the time measurement, is the metastable level
2.sup.3S.sub.1.

6. Atomic clock according to claim 5, characterised in that the
measurement medium is composed exclusively of helium 3, having a level of
1 part per million of atoms taken to the metastable level, when the
second exciter device operates.

7. Atomic clock according to claim 1, characterised in that the first
exciter device comprises a laser beam, and the magnetic fields applied by
the device comprise at least one oscillating magnetic field.

8. Atomic clock according to claim 7, characterised in that the magnetic
fields applied by the device comprise two mutually perpendicular
oscillating magnetic fields.

9. Atomic clock according to claim 7, characterised in that the
essentially static magnetic field is precisely oriented in relation to
the oscillating magnetic field or to the oscillating magnetic fields.

10. Atomic clock according to claim 1, characterised in that it comprises
a magnetic shielding that surrounds it.

Description:

[0002]The subject of the invention is an atomic clock operating with
helium 3.

BACKGROUND

[0003]Atomic clocks comprise a gaseous medium, often alkaline, a device
for exciting the atoms of this gas such as a laser, capable of making
them jump to higher energy states, and means for measuring a frequential
signal emitted by the atoms on returning to the normal energy level,
using the photons coming from the laser.

[0004]The frequency of the signal of the photons returned by the gas is
defined by the formula ν=ΔE/h, where ν is the frequency,
ΔE the difference between the energy levels and h Planck's
constant, equal to 6.62×10-34 J/s.

[0005]It is known that this frequency is very stable and that it can thus
serve as time reference unit. This is however no longer true when the
Zeeman structure of the material is considered: the energy levels then
appear as composed of sub-levels corresponding to slightly different
states, which are distinguished by their angular momentum index mF,
0 for a reference state of the energy level and -1, -2, etc. or +1, +2,
etc. for the others. This is illustrated by FIG. 1 in the case of the
element 87Rb, the breakdown of the first two energy levels (of
angular momentums F=1 and F=2) of which is shown.

[0006]The energy levels are sensitive to the ambient magnetic field. This
sensitivity is low (of the second order) for the sub-level of angular
momentum equal to 0, but much higher (of the first order) for the other
sub-levels: the transitions made from or up to them produce photons, the
frequency of which is variable and thus cannot serve as reference, and
only the portion of the signal corresponding to the transition between
the two sub-levels of zero angular momentum is exploited for the
measurement, which adversely affects its quality. The reference frequency
given by the clock is then fo=EO/h, where E0 is the energy
difference between the sub-levels at mF=0 of the two states (F=1 and
F=2 of the example of FIG. 1).

[0007]Alkaline gases have been preferred until now as measurement medium
in atomic clocks since they generally comprise stable and excited states
each provided with a sub-level with zero angular momentum that thus
ensures a measurement at a stable resonance frequency. These bodies
nevertheless have the drawback of being able to have several physical
states at the ordinary operating conditions and to be chemically very
reactive.

[0008]If it is possible to maintain the ambient magnetic field at a fixed
value, all of the sub-levels are fixed and can contribute to the
measurement. Several techniques for stabilising the ambient magnetic
field have been developed and disclosed in certain publications, such as
American patent US2007/0247241.

SUMMARY

[0009]The object of the invention is to improve existing clocks.

[0010]It is based on the use as measurement medium of helium 3, but which
has been taken to the plasma state by an exciter device separate from the
traditional device serving to excite the particles for the measurement.

[0011]Only gaseous measurement media are generally considered for
measurements in atomic clocks. The use of a plasma, and more specifically
that of helium 3, makes it possible to populate a metastable level
provided with a hyperfine structure, of high frequency, and thus
providing a basis for time measurement appreciable for its precision.

[0012]In addition, since helium 3 is chemically inert, no reaction with
the surrounding material is to be feared; and since only a reduced
portion is usually taken to the plasma state, the greater part remains
gaseous and serves as buffer gas in order to limit the impacts between
the atoms of helium 3 in the metastable level, said atoms being carriers
of the magnetic information.

[0013]A synthetic definition of the invention is an atomic clock
comprising a cell filled with a measurement medium, a first device (1)
for exciting particles of the measurement medium up to a higher energy
level, a system (4, 6, 7) collecting a light energy frequency returned by
the measurement medium on leaving the higher energy level, said light
energy frequency band being exploited to give a time measurement, a
device (9) for applying magnetic fields comprising at least one
essentially static magnetic field and means for controlling (8) said
device (9) to adjust the magnetic fields, characterised in that the
measurement medium comprises helium 3 plasma, and a second exciter device
(10) is provided to give rise to helium 3 plasma from gaseous helium 3.

[0014]The second exciter device may be a "power" radiofrequency wave
generator. The expression signifies that the power that this second
device establishes in the measurement medium is markedly greater than
that which is established by the first exciter device, responsible for
the excitation at the origin of the measurement.

[0015]The radiofrequency waves may be between 20 MHz and 30 MHz, and their
power may be 1 W for a quantity of helium 3 gas of 100 mm3 at a
pressure of around 0.1 Torr. It is sufficient in reality to ionise only a
part of the measurement medium, having for example a level of 1 part per
million of atoms taken to the metastable level, the rest of the helium 3
remaining in the gaseous state and then being without direct utility for
the measurement; it serves however as buffer gas to the atoms of helium 3
in the metastable level. The chemical stability of this element has
already been mentioned, which makes it all the more interesting as buffer
gas given that since it is of the same chemical nature as the element
serving for the measurement it does not react chemically with it, which
is not the case with alkaline gases, which often have to be mixed with
buffer gases to give a stable state. According to a favoured embodiment
of the invention, the measurement medium is, consequently, composed
exclusively of helium 3, the metastable state being the level
23S1.

[0016]Among other solutions, the first exciter device may comprise a laser
beam; and the magnetic fields applied by the device, which are intended
for the stabilisation of the energy levels of the measurement medium, may
comprise at least one essentially static and controlled magnetic field,
and if necessary one or two oscillating magnetic fields perpendicular to
the previous field.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]The invention will now be described with reference to the figures:

[0018]FIG. 1 illustrates an energy diagram of a measurement element in an
atomic clock;

[0019]FIG. 2 is a representation of the atomic clock according to the
invention; and

[0020]FIG. 3 and FIG. 4 illustrate a stabilisation magnetic field control
embodiment.

DETAILED DESCRIPTION

[0021]The core of the clock (FIG. 2) is a cell 1 filled with a measurement
medium. An exciter 2 transmits energy to this medium in the form of a
flux of photons polarised by a quarter wave plate 3. The exciter may be a
laser injecting a light beam to detect the resonances of the medium. A
photodetector 4 collects the light energy returned by the excited medium
of the cell 1 and transmits a signal to a counting device 5, the
photodetector 4 being arranged advantageously in the extension of a laser
beam emanating from the exciter 2. A frequency separator 6 collects the
signal at the output of the counting device 5 and transmits its results
to a device for operating 7 the clock and a control device 8, which
governs the exciter 2 and a device for applying a magnetic field 9.

[0022]There is also a second excitation device 10 to obtain helium 3
plasma from helium 3 gas.

[0023]Herewith several construction components of a possible embodiment of
the invention. The first exciter 2 is a laser diode of wavelength 1083 nm
for a power of 100 mW, with a pumping current modulated to around 3.37
GHz in order to induce an optical intensity modulation charged with
generating the microwave resonance of the hyperfine transition of the
helium 3. The quarter wave plate 3 imposes a left circular polarisation
for the photons. The cell 1 is filled with helium 3 subjected to a
pressure of around 0.1 torr. It is cylindrical, made of Pyrex, and its
volume is 100 mm3. The second exciter device 10 comprises two
electrodes juxtaposed on either side of the cell 1 which are connected to
a power generator of radiofrequencies at 25 MHz (between around 20 MHz
and 30 MHz) and 1 W. It creates the helium plasma, which is necessary to
populate the metastable level 23S1 having the hyperfine
structure.

[0024]The device for applying a magnetic field 9 makes it possible to
apply a magnetic field HO of 500 μT parallel to the laser beam to
block the sub-levels at constant energies. A pair of Helmoltz coils is
used for this. This magnetic field is controlled to a constant value by
the measurement of the Larmor frequency within the hyperfine structure.
In this way, variations in the ambient magnetic field are prevented from
perturbing the transition of microwaves defining the resonance frequency
f0.

[0025]The device for applying a magnetic field 9 again generates a
component of oscillating magnetic field at low frequency, applied
perpendicular to the static magnetic field and which is controlled thanks
to the control device 8 at the Zeeman transition at around 12 MHz. This
oscillating field makes it possible to induce a resonance within Zeeman
sub-levels, which will give the abovementioned measurement to evaluate
the resulting ambient magnetic field and control it to a constant value.

[0026]Since helium 3 is not provided with sub-levels with zero angular
momentum index, it is necessary to make the device operate at constant
magnetic field, which may be obtained by a controlled artificial field
with or without magnetic shielding. The control of the magnetic field may
be accomplished in a scalar or vectorial manner by the Larmor or
vectorial frequency by a zero total magnetic field search.

[0027]The device for applying a magnetic field 9 may at the same time
generate the magnetic field serving for the resonance measurement if it
is composed of controlled triaxial coils.

[0028]In an improved conception, the device for applying a field 9 emits
magnetic fields at radiofrequencies of pulsations noted Ω and
ω, which are mutually perpendicular and of direction dependent on
the polarisation (for example perpendicular to the light rays emitted by
the exciter 2 in the case of a circular polarisation).

[0029]Reference is made to FIG. 3. The signal coming from the counting
device 5 comprises several light rays, and firstly one which is at the
useful frequency f0 corresponding to the restitution of the photons
by the gaseous medium and which gives the reference to the time
measurement. It again shows spectral lines at the frequencies
Ω/2π, (ω-Ω)/2π, ω/2π, and
(ω+Ω)/2π. These spectral lines appear for magnetic fields
of low values, well below 1/δ.TR, where TR is the
relaxation time of the sub-levels and γ is their gyromagnetic
ratio, characteristic of the chemical element excited. They correspond to
resonances between the sub-levels. Their amplitude is proportional to the
ambient magnetic field. It is in keeping with this method of control to
apply a magnetic field for compensating the essentially static ambient
magnetic field, but which is varied in a continuous manner in amplitude
and in direction if necessary, so that the amplitude of these lines is
reduced as much as possible, which signifies that the compensation field
has balanced out the ambient magnetic field. FIG. 4 then shows that the
sub-levels of each principal level are at a same energy value, so that
the photons returned by the gaseous medium are all at the useful
frequency f0: the corresponding spectral line appears in the form of
a much sharper and higher peak, the detection of which is thus
facilitated. It becomes conceivable to omit the traditional magnetic
shielding of atomic clocks; however, since the magnetic shielding filters
the electric field by skin effect, an electric shielding is
advantageously added so as not to disrupt the energy levels of the atoms
if the magnetic shielding is eliminated. The amplitudes of the
radiofrequency fields are advantageously chosen to maximise the amplitude
of the spectral resonance lines (before the application of the
compensation static field). It is recommended to approximately respect
the equalities γHω/ω=1 and γHΩ/Ω=1,
where Hω and HΩ are the amplitudes of the radiofrequency
fields of pulsations ω and Ω. Advantageously, the device for
applying the magnetic field 9 applies at the same time the substantially
static compensation magnetic field and the radiofrequency magnetic
fields.

[0030]It may consist of triaxial coils, or three mutually concentric
monoaxial coils. The control is accomplished by any known material
comprising a computing unit. The coils are current or voltage driven. The
excitation to the resonance frequency f0 is accomplished by an
amplitude modulation of the laser diode at the frequency f0/2 or by
a microwave cavity resonating at the frequency f0. An exciter
comprising two lasers, the difference in frequency of which is f0,
may also be envisaged.

[0031]Since helium 3 is not provided with sub-levels with zero angular
momentum index, it is necessary to make the device operate at constant
magnetic field, which may be obtained by a controlled artificial field
with or without magnetic shielding. The control of the magnetic field may
be accomplished in a scalar or vectorial manner by the Larmor or
vectorial frequency by a zero total magnetic field search.

[0032]The device for applying a magnetic field 9 may at the same time
generate the magnetic field serving for the measurement of the resonance
if it is composed of controlled triaxial coils.

[0033]The instrument measuring the laser flux may be a photodiode of
InGaAs type. This embodiment, comprising a device for stabilising the
magnetic field, does not comprise magnetic shielding. However, it is also
possible to use a magnetic shielding in addition to the device for
controlling the magnetic field as described previously. The magnetic
shielding may be composed for example of a cylinder of soft iron and a
cylinder of overlapping μ metal.

[0034]The exciter 2 could comprise a lamp or a VCSEL (for variation
capacity surface emitting light). In the absence of a device for
stabilising the ambient magnetic field, the excitation to the resonance
frequency could also be brought about by a microwave resonating cavity or
by two lasers, the frequency difference of which is the resonance
frequency.